This invention relates to a method and apparatus for injecting a temperature sensitive probe into a tire to be cured. More particularly, this invention relates to such a probe injection system in a tire curing press of the type wherein the mold is heated by steam in a cavity surrounding the mold.
At one time the curing or vulcanization of pneumatic tires has been controlled purely as a function of time; that is, the time a tire had to be heated to be satisfactorily cured and the temperature of the cure were empirically determined based on the particular rubber compound, size of the tire and other like factors. While such a time cure control proved satisfactory for years, it was recognized that not every tire in a batch is the same nor are the heating characteristics of a tire press the same from cure to cure. Thus, when an arbitrary time for all cures of a particular type of tire was selected, some were overcured while others undercured. Such unprecise curing not only resulted in tires of lesser grade, and sometimes scrap, but it also represented a wasting of valuable time of the capital equipment -- the tire curing press.
The first attempts to optimize the cure of a tire utilized a system whereby the temperature at the boundaries of the tire, that is, at the mold surface and internally of the tire, were measured and recorded during the cure. Then the critical temperature, that is, the temperature internally of the tire, was calculated by use of known heat transfer characteristics of the tire. An example of such a system is shown in Davis, et al. U.S. Pat. No. 3,649,729. This attempt to calculate rather than measure the temperature internally of the tire resulted in inaccuracies at least equivalent to those of a pure time control, described hereinabove, primarily due to the inponderables involved in the constantly changing heat transfer characteristics.
Gould, et al. U.S. Pat. No. 3,718,721 represents the next attempt at optimizing the cure of a pneumatic tire. There, a temperature sensing probe, such as a thermocouple, was permanently mounted in the tire mold with the closing of the mold and shaping of the tire injecting the probe into the tire. It was the theory in this instance to overcome the problems of the boundary sensors by obtaining temperature data right at the site internally of the tire. The problem with this system was that the fixed probe had to withstand the forces of loading and shaping the uncured tire and unloading the cured tire. To do so, a fairly heavy gauge probe was required. Such a probe was susceptible to false temperature readings in that mold heat was conducted by the probe body to the probe sensor. Smaller probes were less susceptible to such inaccuracies but were often damaged by the curing forces previously described.
The most recent and only really successful efforts in obtaining true data internally of a tire and thereafter controlling the cure of the tire is found in the disclosures of Smith U.S. Pat. Nos. 3,819,915; 3,980,743; and 4,022,555. According to these concepts, a temperature sensitive probe, preferably a thermistor, is injected into the tire after the mold has been closed and the tire shaped. Most often a double-acting air cylinder is utilized to inject and withdraw the probe. Such operation is entirely satisfactory for use with vulcanizing molds heated by conventional platens in that there is access to the outside of the molds for mounting the air cylinder and enough clearance for operating the cylinder. However, many larger tires are cured in a press wherein the molds are encased in a "steam dome" or cavity which receives steam under pressure. In this instance space limitations often create a problem in that because of the larger tires with attendant longer cylinder stroke for deeper probe injection, it is often difficult to mount the cylinder with suitable clearances within the steam dome, as is required in order to have access to the mold. In addition, mounting such a double-acting cylinder in the steam dome exposes the same to the environment of 100 psi saturated steam thereby requiring the use of an expensive, well-sealed cylinder. Finally, using a double-acting cylinder in a steam dome press required two access holes in the steam dome with attendant heavy seals for the air supply hoses which gives rise to higher installation and maintenance costs.